1. Field of the Invention
The present invention relates to piezoelectric ceramics broader applicable in fields of resonators or pressure sensors to be used at high temperature.
2. Description of the Related Art
A piezoelectric substance is such a material having a piezoelectric effect where electric polarization is changed by receiving external stress and a reverse piezoelectric effect where strain is generated by impressing an electric field. It has been applied in sensors, resonators or actuators for measuring pressure or deformations.
Most of piezoelectric materials are now practiced generally in ferroelectrics having perovskite structures of PZT group (PbZrO3--PbTiO3solid solution) of tetragonal system or rhombic system or of PT (PbTiO3) group of tetragonal system. By adding various sub-elements thereto, responses to many requisite characteristics may be designed. For example, for position adjusting actuators requiring large displacing amounts in using as D.C., such piezoelectrics are utilized where coefficient (Qm) of mechanical quality is small but piezoelectric constant (d33) is large. Alternatively, for using as AC as ultrasonic wave generating elements to be used in an ultrasonic motor, such piezoelectrics are utilized where piezoelectric constant (d33) is small but coefficient (Qm) of mechanical quality is large.
However, the PZT or PT based piezoelectric substances are around 200 to 400.degree. C. of the curie point in practical compositions, and at temperature exceeding these points, the substances become paraelectric and the piezoelectricity lapses. Therefore, for utilization at high temperature, or for example, in nuclear reactor controlling sensors, the substances cannot be applied. Further, since these lead based piezoelectrics much (around 60 to 70 wt %) contain lead oxides (PbO) of very high volatility even at low temperature, they are not preferable from a biological view and preventing phases of environmental pollution. Actually, when the lead based piezoelectric materials are manufactured as ceramics or monocrystals, heat treatments as calcination or melting are indispensable, and when considering at industrial production levels, lead oxides of volatile elements very much volatilize or disperse in an atmospheric air. Lead oxide issued during manufacturing can be recovered, but lead oxide contained in the piezoelectrics as industrial production sales can scarcely recover, and when they are discharged widely, the environmental pollution is inevitably caused at the present time.
As piezoelectrics not containing lead at all, well known is, for example, BaTiO3of the perovskite structure belonging to the tetragonal system, but this curie point is low as 120.degree. C. and not practical. JP-A-9-100156 (hereinafter, JP-A means "Unexamined Japanese Patent Publication (kokai)" describes (1-x)(Bi1/2Na1/2) TiO3-xNaNbO3solid solution, but does not describe substances exceeding the curie of 370.degree. C., and thus still impossible to apply to elements at ultra high temperature as nuclear reactor controlling sensors.
As piezoelectrics enabling to bring up the curie point exceedingly 500.degree. C., known is, for example, a compound in bismuth layer. However, the bismuth layer like compound containing no lead has problems that Qm and Qmax important, when applied to the resonators are small. That Qm becomes small is considered that coercive field is large so that polarization is not enough. Qmax is meant by tan .theta.max when a maximum value of phase angle is Omax, that is, when X is reactance and R is resistance, it is a maximum value of Q (=.vertline.x.vertline./R) between resonant frequency and anti-resonant frequency. The larger Qmax, the more oscillation is stable, and oscillation at low voltage is possible. JP-A-6-305817 describes, as the bismuth layer compound, PbxBi3-xTi1-xNb1+xO9(0.3.ltoreq.x.ltoreq.0.75). In this composition, the curie temperature exceeds 500.degree. C., but according to inventors' experiments, enough large Qm and Qmax cannot be obtained, and a problem of "containing lead" remains. JP-A-54-32957 describes piezoceramics of ferroelectricity containing at least one kind of Mn, Fe, Co, Ni and Cr in the sintered matter having the bismuth layer shaped structure shown with (Pb1-xSrx)Bi4Ti4O15(on condition of 0&lt;x&lt;1), but this structure also contains lead.